Thermal interface materials (TIMs) are critical to protect active semiconductor devices, such as microprocessors, from exceeding the operational temperature limit. They enable thermal bonding of the heat generating device (e.g., a silicon semiconductor) to a heat sink or a heat spreader (e.g. copper and/or aluminum components) without presenting an excessive thermal barrier. Different TIMs may also be used in the assembly of other components of the heat sink or the heat spreader stack that comprise the overall thermal impedance path.
Formation of a small thermal barrier is an important property of a TIM. The thermal barrier can be described in terms of effective thermal conductivity through the TIM and is preferably as high as possible. The effective thermal conductivity of the TIM is primarily due to the interfacial heat transfer coefficient as well as the (intrinsic) bulk thermal conductivity of the TIM. A variety of other properties are also important for a TIM depending on the particular application, for example: an ability to accommodate or avoid thermal expansion stresses when joining two materials, an ability to form a mechanically sound joint that is stable during thermal cycling, a lack of sensitivity to moisture and temperature changes, manufacturing feasibility, and cost.
Several classes of materials are being currently used as TIMs, for example, thermal greases, thermal gels, adhesives, elastomers, thermal pads, and phase change materials. Although the foregoing TIMs are adequate for many current semiconductor devices, the increased performance of semiconductor devices in the near future will render the presently known TIMs inadequate. Specifically, the thermal conductivity of current non-metallic TIMs generally does not exceed about 5 W/mK and is typically less than about 1 W/mK. However, TIMs that form thermal interfaces with effective thermal conductivities of about 50 W/mK or greater will soon be needed.
One alternative to the foregoing non-metallic TIMs is a solid metal sheet or preform made of a typical solder alloy. The metal TIMs ensure high thermal conductivity value (e.g., about 80 W/mK for an indium sheet). Metal TIMs may also exhibit a favorable solder or wetting behavior upon reflow which facilitates a low thermal interfacial resistance. During reflow, the solder and substrate are heated, the solder melts and wets by surface tension and/or local surface alloying. The interfaces consist of intermetallics or interdiffused metals with thermal properties that are frequently less desirable than those of the bulk TIM metal but much better than existing (polymer based) TIMs. In most cases, metallic TIMs have to be subjected to reflow in order to form reliable thermal interfaces. Metallic TIMs, however, can fail in certain applications due to the relatively large difference(s) between the coefficients of thermal expansion (CTEs) of the TIM and the semiconductor and/or heat sink components and the lack of compliance.